MDX and DCX are not supported in any official capacity and never were. They are third-party modifications of mIRC. If you choose to use them, you should ASSUME that your scripts will break in every release of mIRC. It has nothing to do with mIRC itself. If you used only standard mIRC scripting controls, you wouldn't have that problem.
mIRC is breaking compatibility with versions of Windows prior to XP because doing so was necessary to support new features like full Unicode support. If you must continue to use an older version of Windows, then you can just continue to use 6.35. You wouldn't benefit from most of the new features in 7.0 anyway since they simply wouldn't work in Windows ME or earlier.
